[applets/systemtray] Fix highlight when applet comes back from hidden

Highlight should work correctly if selected applet changes state
and is moved to hidden area and then comes back to visble area.

BUG: 435687
FIXED-IN: 5.22.0

applets/systemtray/package/contents/ui/CurrentItemHighLight.qml

@@ -54,6 +54,7 @@ PlasmaCore.FrameSvgItem {
             return prefix;
     }
 
+    // update when System Tray is expanded - applet activated or hidden icons shown
     Connections {
         target: systemTrayState
 
@@ -66,6 +67,7 @@ PlasmaCore.FrameSvgItem {
         }
     }
 
+    // update when applet changes parent (e.g. moves from active to hidden icons)
     Connections {
         target: systemTrayState.activeApplet
 
@@ -74,6 +76,7 @@ PlasmaCore.FrameSvgItem {
         }
     }
 
+    // update when System Tray size changes
     Connections {
         target: parent
 
@@ -86,9 +89,18 @@ PlasmaCore.FrameSvgItem {
         }
     }
 
+    // update when scale of newly added tray item changes (check 'add' animation in GridView in main.qml)
+    Connections {
+        target: !!highlightedItem && highlightedItem.parent ? highlightedItem.parent : null
+
+        function onScaleChanged() {
+            updateHighlightedItem();
+        }
+    }
+
     function updateHighlightedItem() {
         if (systemTrayState.expanded) {
-            if (systemTrayState.activeApplet && systemTrayState.activeApplet.parent.inVisibleLayout) {
+            if (systemTrayState.activeApplet && systemTrayState.activeApplet.parent && systemTrayState.activeApplet.parent.inVisibleLayout) {
                 changeHighlightedItem(systemTrayState.activeApplet.parent.container);
             } else { // 'Show hiden items' popup
                 changeHighlightedItem(parent);
@@ -100,8 +112,8 @@ PlasmaCore.FrameSvgItem {
 
     function changeHighlightedItem(nextItem) {
         // do not animate the first appearance
-        // or when size of parent (root) changes
-        if (!highlightedItem || (highlightedItem === nextItem && nextItem === parent)) {
+        // or when the property value of a highlighted item changes
+        if (!highlightedItem || (highlightedItem === nextItem)) {
             animationEnabled = false;
         }